This invention relates in general to hybrid drive systems, such as are used in conjunction with drive train assemblies for vehicles. In particular, this invention relates to an improved apparatus and method for operating a hybrid drive system in such a manner as to reduce the speed of the vehicle during an extended braking condition, such as might occur when a vehicle is driven down a lengthy hill.
Drive train assemblies are widely used for generating power from a source and for transferring such power from the source to a driven mechanism. Frequently, the source generates rotational power, and such rotational power is transferred from the source of rotational power to a rotatably driven mechanism. For example, in most land vehicles in use today, an engine generates rotational power, and such rotational power is transferred from an output shaft of the engine through a driveshaft to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle.
In some of these land vehicles and other mechanisms, a hybrid drive system (also known as an energy recovery system) is provided in conjunction with the drive train assembly to selectively decelerate the rotatably driven mechanism, accumulate energy resulting from such deceleration, and use the accumulated energy to subsequently re-accelerate the rotatably driven mechanism. To accomplish this, a typical hybrid drive system includes a reversible energy transfer machine that is coupled to the drive train assembly and an energy storage device that communicates with the reversible energy transfer machine. To decelerate the vehicle, the hybrid drive system functions in an energy recovery mode, wherein the reversible energy transfer machine slows the rotation of the rotatably driven mechanism and stores the kinetic energy of the vehicle in the energy storage device as potential energy. To subsequently re-accelerate the vehicle, the hybrid drive system functions in a driving mode, wherein the potential energy stored in the energy storage device is supplied to the reversible energy transfer machine to rotatably drive the rotatably driven mechanism.
Virtually all land vehicles are provided with conventional foundation brakes that can be manually operated by a driver to reduce the speed of the vehicle. On those vehicles that are additionally provided with a hybrid drive system, the use of such foundation brakes can be significantly reduced because the hybrid drive system can decelerate the vehicle during the energy recovery mode described above. However, the ability of the hybrid drive system to recover and store energy in this manner is limited. Thus, when braking occurs for an extended period of time (such as when the vehicle is descending a lengthy hill), it is undesirable for a conventional hybrid drive system to be operated in the normal energy recovery mode to decelerate the vehicle. Consequently, the foundation brakes of the vehicle must be applied for an extended period of time, which can generate undesirable heat. Accordingly, it would be desirable to provide an improved apparatus and method for operating a hybrid drive system in such a manner as to reduce the speed of the vehicle during an extended braking condition.